On 9/23/19 2:48 PM, Jens Axboe wrote:
> On 9/23/19 10:32 AM, Pavel Begunkov wrote:
>> Sorry, mixed the threads.
>>
>>>>
>>>> I'm not sure an extension is needed for such a special interface, why not
>>>> just put a ->threshold value next to the ctx->wait field and use either
>>>> the regular wait_event() APIs with the proper condition, or
>>>> wait_event_cmd() style APIs if you absolutely need something more complex
>>>> to happen inside?
>> Ingo,
>> io_uring works well without this patch just using wait_event_*() with
>> proper condition, but there are performance issues with spurious
>> wakeups. Detailed description in the previous mail.
>> Am I missing something?
>
> I think we can do the same thing, just wrapping the waitqueue in a
> structure with a count in it, on the stack. Got some flight time
> coming up later today, let me try and cook up a patch.
Totally untested, and sent out 5 min before departure... But something
like this.
diff --git a/fs/io_uring.c b/fs/io_uring.c
index ca7570aca430..c2f9e1da26dd 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2768,6 +2768,37 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit,
return submit;
}
+struct io_wait_queue {
+ struct wait_queue_entry wq;
+ struct io_ring_ctx *ctx;
+ struct task_struct *task;
+ unsigned to_wait;
+ unsigned nr_timeouts;
+};
+
+static inline bool io_should_wake(struct io_wait_queue *iowq)
+{
+ struct io_ring_ctx *ctx = iowq->ctx;
+
+ return io_cqring_events(ctx->rings) >= iowq->to_wait ||
+ atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
+}
+
+static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
+ int wake_flags, void *key)
+{
+ struct io_wait_queue *iowq = container_of(curr, struct io_wait_queue,
+ wq);
+
+ if (io_should_wake(iowq)) {
+ list_del_init(&curr->entry);
+ wake_up_process(iowq->task);
+ return 1;
+ }
+
+ return -1;
+}
+
/*
* Wait until events become available, if we don't already have some. The
* application must reap them itself, as they reside on the shared cq ring.
@@ -2775,8 +2806,16 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit,
static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
const sigset_t __user *sig, size_t sigsz)
{
+ struct io_wait_queue iowq = {
+ .wq = {
+ .func = io_wake_function,
+ .entry = LIST_HEAD_INIT(iowq.wq.entry),
+ },
+ .task = current,
+ .ctx = ctx,
+ .to_wait = min_events,
+ };
struct io_rings *rings = ctx->rings;
- unsigned nr_timeouts;
int ret;
if (io_cqring_events(rings) >= min_events)
@@ -2795,15 +2834,16 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
return ret;
}
- nr_timeouts = atomic_read(&ctx->cq_timeouts);
- /*
- * Return if we have enough events, or if a timeout occured since
- * we started waiting. For timeouts, we always want to return to
- * userspace.
- */
- ret = wait_event_interruptible(ctx->wait,
- io_cqring_events(rings) >= min_events ||
- atomic_read(&ctx->cq_timeouts) != nr_timeouts);
+ iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
+ prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
+ do {
+ if (io_should_wake(&iowq))
+ break;
+ schedule();
+ set_current_state(TASK_INTERRUPTIBLE);
+ } while (1);
+ finish_wait(&ctx->wait, &iowq.wq);
+
restore_saved_sigmask_unless(ret == -ERESTARTSYS);
if (ret == -ERESTARTSYS)
ret = -EINTR;
--
Jens Axboe
